The present invention relates to an apparatus for transporting a carriage along a path. The invention particularly relates to such an apparatus used to transport a printhead along a line whereon characters are to be printed in a typewriter or printer.
It is known in the typewriter and computer controlled printing machines arts to transport a printing module or printhead along a line of printing, adjacent to a platen. Such printers typically employ dot matrix, daisy wheel or thermal devices to print upon paper held against the platen.
In use, the printhead is transported along the line of print such that adjacent characters printed on the paper are properly spaced apart. This transportation of the printhead, in the prior art, is achieved by means of belts, leadscrews, wires and other devices linked to a printhead carriage constrained by tracks, sliders and other means to move along the printing path. The distance moved by the printhead between successive printing operations is small, and in consequence the quality of the drive mechanism for the printhead carriage is required to be high. In the prior art, it is known to use stepping motors to drive the belts or wire, in which instance it is necessary for the stepping motors to be of high quality and accuracy, and to have many steps of angular position in each full rotation. These prior art solutions have the further disadvantage that the quality of the mechanical parts is required to be extremely high since drive from a motor is applied more or less directly to the printhead carriage.
The present invention seeks to improve over the prior art by providing a printhead carriage which is driven along the printing path using a principle of differential motion whereby gross movements in a wire, belt or rack causes only a small linear displacement of the carriage, allowing coarse and relatively imprecise mechanical parts to be employed. The present invention further seeks to provide improvement over the prior art by allowing for the motor which moves the printhead to be selectably mounted either on the printhead itself or on the body of the printer, thereby making for a compact construction.